a) Field of the Invention
This invention relates to electrically-conductive resin compositions excellent in antistatic properties, and also to electrically-conductive resin moldings or products obtained by molding or otherwise forming the electrically-conductive resin compositions. The expression “moldings or products” may hereinafter be collectively called “moldings”. Likewise, the expression “molding or otherwise forming” may hereinafter be collectively called “molding”.
b) Description of the Related Art
Various electronic parts, for example, ICs, capacitors, transistors and LSIs have conventionally been stored and shipped by sealing them in packages or containers called “electrically-conductive (antistatic) carrier tapes or carrier trays”, as these electronic parts are susceptible to damage by a spark of charged static electricity. These packages or containers for such electronic parts are used by molding or otherwise forming them with an electrically-conductive resin composition or applying an electrically-conductive coating formulation to surfaces of packages or containers molded or otherwise formed with an electrically-insulating resin such that the packages or containers are provided with electrical conductivity to such an extent as preventing the charging of static electricity on them.
As an electrically-conductive material for the above-described electrically-conductive resin composition or electrically-conductive coating formulation, electrically-conductive carbon black is used most widely. However, carbon black is in a fine particulate form. It is, therefore, accompanied by a problem in that carbon black is prone to separation from the surfaces of packages or containers of electronic parts and the fine particles of the carbon black so separated may damage the electronic parts. Further, carbon black is available only in a black color, and therefore, involves another problem in that the packages or containers are inferior in external attractiveness. As electrically-conductive materials capable of replacing electrically-conductive carbon black, a variety of electrically-conductive high molecular materials have been proposed. However, these electrically-conductive high molecular materials are costly and are required to be used in large proportions. They are, hence, inferior in practical utility.
As electrically-conductive materials capable of substituting for such electrically-conductive high molecular materials, dissociable inorganic salts such as lithium perchlorate have also been proposed, for example. However, these dissociable inorganic salts have low compatibility with thermoplastic resins which make up packages or containers or with coating-forming materials in electrically-conductive coating formulations, so that it is difficult to evenly disperse them in the resins or coatings. If they are used in an proportion large enough to provide sufficient electrical conductivity, the dissociable inorganic salts are separated from the packages or containers themselves or from the electrically-conducting coatings, and the thus-separated inorganic salts likewise damage the electronic parts.
Lithium perchlorate, one of the above-described dissociable inorganic salts, is known to be dissolved to a certain proportion in a resin containing ether bonds, for example, polyethylene oxide or polypropylene oxide or in a polyurethane resin formed of polyethylene oxide diol or polypropylene oxide diol as a constituent such that they form a solid solution. However, the solubility of lithium perchlorate is low, so that the use of lithium perchlorate in a proportion large enough to provide sufficient electrical conductivity develops a similar problem as mentioned above, such as damage to electronic parts. An increase in the molecular weight of polyethylene oxide or polypropylene oxide, in which lithium perchlorate is to be dissolved, makes it possible to provide lithium perchlorate with improved solubility. Such an increased molecular weight, however, leads to a reduction in the compatibility with the other resin or the solubility in a solvent upon preparation of a coating formulation, thereby failing to obtain a resin composition or coating formulation having sufficient practical utility.
To resolve such problems as described above, a technique is proposed in JP-A-2001-316593. Proposed is an electrically-conductive resin composition obtained by kneading an electrically-conductive polyurethane resin, which has been prepared by doping with a dissociable inorganic salt a polyurethane resin having polyalkylene oxide segments, with another thermoplastic resin. As the electrically-conductive resin contains hydrophilic polyoxyethylene segments in a large proportion, the electrically-conductive resin may not be able to exhibit sufficient compatibility depending on the kind of the thermoplastic resin. The electrically-conductive resin is, therefore, accompanied by a potential problem that a molding may not be obtained with uniform electrical conductivity on the surface thereof.